Brushless-motor drive apparatus

ABSTRACT

Disclosed is a brushless-motor drive apparatus provided with: a current detecting means ( 6 ) for detecting currents flowing through armature windings ( 9 ) during periods when switching elements ( 5 H,  5 L) of a drive circuit are ON; a calculation processing means for comparing a target current value and the detected current value, and calculating voltage command values to be applied to the armature windings ( 9 ); and a PWM driving means ( 4 ) for controlling the ON/OFF of the switching elements on the basis of the voltage command values. The calculation processing means is further provided with a current-detection possibility evaluating means for evaluating whether or not the currents flowing through the armature windings ( 9 ) can be detected, with the operating states of the switching elements of each of the phases, and when the current-detection possibility evaluating means evaluates that the currents cannot be detected, the calculation processing means obtains the voltage command values using the current values detected when the currents are able to be detected, and continues the motor drive.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a National Stage of International Application No.PCT/JP2010/058207, filed on May 14, 2010, the contents of all of whichare incorporated herein by reference in their entirety.

TECHNICAL FIELD

The present invention relates to a brushless-motor drive apparatusconfigured to control driving of a brushless motor using phase currentsflowing through armature windings of the brushless motor.

BACKGROUND ART

In a motor drive apparatus that performs feedback control on an outputof a brushless motor according to armature currents flowing througharmature windings of the motor, currents flowing through currentdetection resistors provided phase by phase to a motor drive invertercircuit are detected as armature currents in the respective phases. Aduty ratio corresponding to a deviation between a target current and thedetection current is calculated phase by phase and ON/OFF control isperformed on switching elements of the respective phases forming theinverter circuit by PWM control. Owing to this configuration, phasevoltages to be applied to the armature windings of the respective phasesare changed and drive currents are flown through the armature windingsof the respective phases. Hence, a motor output is controlled.

FIG. 14 is a view of a circuit configuration showing an example of abrushless-motor drive apparatus in the related art described, forexample, in Patent Document 1.

As is shown in FIG. 14, the brushless-motor drive apparatus in therelated art is configured in such a manner that upper switching elements5 uH, 5 vH, and 5 wH (hereinafter, also collectively denoted simply by5H), armature windings 9, and lower switching elements 5 uL, 5 vL, and 5wL (hereinafter, also collectively denoted simply by 5L) areinterconnected phase by phase and current detection resistors 6 u, 6 v,and 6 w (hereinafter, also collectively denoted simply by 6) aredisposed between the lower switching elements 5L and a ground. In orderto detect currents flowing through the current detection resistors 6, astate of the switching elements is changed so that the lower switchingelements 5L are ON by a drive control circuit 4 that is a PWM controlmeans. In this instance, armature currents of the armature windings 9 ofthe motor pass through the lower switching elements 5L and the currentdetection resistors 6 and flow to the ground. The armature currents aremeasured by detecting potential differences across the current detectionresistors 6 in this state in which currents flow through the currentdetection resistors 6.

When the lower switching elements 5L are ON, the upper switchingelements 5H are OFF so that currents are prevented from flowing throughthe upper and lower switching elements of the same phases by flowingcurrents through the armature windings 9. In this manner, the armaturecurrents can be detected only when the upper switching elements 5H areOFF and the lower switching elements 5L are ON.

Conversely, in a case where ON/OFF control is performed on the switchingelements 5H and 5L with PWM control signals of the drive control circuit4, there is a period during which no currents flow through the currentdetection resistors 6. Accordingly, there is a circumstance wherecurrents cannot be detected properly. To overcome this inconvenience, itis necessary to limit duty ratios of the PWM control signals, that is,voltages to be applied to the armature windings 9. Hence, the motor hasto be used by lowering a utilization ratio of a power supply voltage dueto the duty ratios and a motor performance is limited.

Under these circumstances, Patent Document 1 describes as follows. Thatis, for a brushless motor formed of a phase U, a phase V, and a phase W,in a case where ON/OFF control is performed on switching elements of therespective phase U, phase V, and phase W according to instructions ofthe duty ratios from the PWM control means, it becomes difficult todetect a current in the phase U when the switching elements of the phaseU are switched ON/OFF at a duty ratio lower than a predetermined value.Hence, a current Iu in the phase U is calculated in accordance withEquation (1) below from a detected current Iv in the phase V and adetected current Iw in the phase W:Iu=−(Iv+Iw)  (1).

Also, it is described that in a case where the switching elements of theother phase V or W are switched ON/OFF at a duty ratio lower than thepredetermined value, a current is calculated .using the currentsdetected in the other phases in the same manner.

RELATED ART LIST Patent Document

Patent Document 1: Japanese Patent No. 4140454

SUMMARY OF THE INVENTION Problems to be Solved by the Invention

In the related art described above, when a duty ratio of a given phaseis lower than the predetermined value, for example, in a state in whicha duty ratio of the phase U is lower than the predetermined value, aU-phase current cannot be detected directly. Accordingly, the U-phasecurrent is calculated by detecting currents in the other phases, thatis, V-phase and W-phase currents. It should be noted, however, that theother phases, the phase V and the phase W, in this instance have to bein a state in which currents can be detected without fail. In otherwords, it is required that duty ratios of two phases out of three phasesbe equal to or higher than the predetermined value. Hence, duty ratiosare limited to a range within which currents can be detected and thislimitation raises a problem that duty ratios cannot be controlled asdesired.

The invention is devised to solve the problems as discussed above andhas an object to provide a brushless-motor drive apparatus capable ofdriving a motor independently of a state in which armature currents canbe detected and enhancing a motor output by eliminating a need to limitduty ratios and hence by becoming capable of applying voltages toarmature windings of the motor as desired.

Means for Solving the Problems

A brushless-motor drive apparatus of the invention is provided with: adrive circuit of a brushless motor having a plurality ofparallel-connected arms including switching element pairs formed offirst (upper) switching elements disposed on a power supply side andsecond (lower) switching elements disposed on a ground side connected inseries in each pair; a current detecting means for detecting currentsflowing through armature windings of the brushless motor during periodswhen the switching elements of the drive circuit are ON; a calculationprocessing means for comparing a target current value to drive thebrushless motor and detection current values detected by the currentdetecting means, and calculating voltage command values to be applied tothe armature windings of the brushless motor on the basis of deviations;and a PWM driving means for controlling ON/OFF of the switching elementsof the drive circuit by generating drive signals on the basis of thevoltage command values from the calculation processing means. Thecalculation processing means is further provided with acurrent-detection possibility evaluating means for evaluating whether ornot the currents flowing through the armature windings of respectivephases of the brushless motor can be detected according to operatingstates of the switching elements of the respective phases of the drivecircuit. When the current-detection possibility evaluating meansevaluates that the currents cannot be detected, the calculationprocessing means obtains the voltage command values by making acomparison with the target current value using detection current valuesdetected before and continues a motor drive.

Advantage of the Invention

According to the brushless-motor drive apparatus of the invention, itbecomes possible to obtain a brushless-motor drive apparatus capable ofdriving the motor independently of a state in which armature currentscan be detected and enhancing a motor output by eliminating a need tolimit duty ratios and hence by increasing a utilization ratio of a powersupply voltage.

The foregoing and other objects features, aspects, and advantages of thepresent invention will become more apparent from the following detaileddescription of the present invention when taken conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view schematically showing a configuration of abrushless-motor drive apparatus according to a first embodiment of theinvention.

FIG. 2 is a time chart used to describe an operation of currentdetecting means in the first embodiment of the invention.

FIG. 3 shows driving of an upper FET in the first embodiment of theinvention.

FIG. 4 shows driving of a lower FET in the first embodiment of theinvention.

FIG. 5 is a detailed control block diagram of a coordinate convertingmeans in the first embodiment of the invention.

FIG. 6 is a flowchart depicting processing of current detection in thefirst embodiment of the invention.

FIG. 7 is a flowchart depicting processing at the occurrence of anabnormality in the motor drive apparatus according to the firstembodiment of the invention.

FIG. 8 is a view showing a motor characteristic and a current detectablerange in the first embodiment of the invention.

FIG. 9 is a detailed control block diagram of a coordinate convertingmeans according to a second embodiment of the invention.

FIG. 10 is a flowchart depicting processing of current detection in thesecond embodiment of the invention.

FIG. 11 is a view showing a motor angle and a current detectable rangein the second embodiment of the invention.

FIG. 12 is a flowchart depicting processing at the occurrence of anabnormality in the motor drive apparatus according to the secondembodiment of the invention.

FIG. 13 is a flowchart depicting another example of the processing atthe occurrence of an abnormality in the motor drive apparatus accordingto the second embodiment of the invention.

FIG. 14 is a view schematically showing a configuration of an example ofan apparatus in the related art.

MODE FOR CARRYING OUT THE INVENTION

First Embodiment

Hereinafter, a brushless-motor drive apparatus according to a firstembodiment of the invention will be described with reference to thedrawings. FIG. 1 is a view schematically showing a configuration of thebrushless-motor drive apparatus of the first embodiment. Referring toFIG. 1, a brushless motor is formed of a three-phase synchronouspermanent magnet motor having armature windings 9 and includes aposition sensor 10 formed, for example, of a resolver, that detects amagnetic pole position of the motor. Magnetic pole position informationof the motor is outputted by the position sensor 10 to a control portion(controller) described below.

A drive circuit that drives the brushless motor is formed of, as isknown, FETs (Field Effect Transistors) 5H (5 uH, 5 vH, and 5 wH) thatare first switching elements disposed on a side of a power supply 8 andFETs 5L (5 uL, 5 vL, and 5 wL) that are second switching elementsdisposed on a grounding side, which are connected in series in pairs.Connection points of the FETs 5H and 5L in respective pairs areconnected to respective phases of the armature windings 9 of the motor.Conduction and non-conduction of the respective FETs 5H and 5L arecontrolled according to drive signals from a PWM drive means 4, so thatthe motor is driven by controlling applied voltages to the motorarmature windings 9.

Current detection resistors 6 (6 u, 6 v, and 6 w) that are currentdetecting means are connected between the lower FETs 5 (5 uL, 5 vL, and5 wL) and the ground. A specific operation of current detection will bedescribed below.

A calculation processing means that is a control portion of the motor isformed of a micro-computer and calculates a motor position θ upon inputof a signal from the position sensor 10 that detects a rotation positionof the motor.

Also, currents flowing through the respective phases U, V, and W of thearmature windings 9 are inputted from the current detecting means 6 thatdetect currents flowing through the respective phases of the brushlessmotor.

Hereinafter, respective portions of the calculation processing meanswill be described.

Currents Iu, Iv, and Iw from the current detecting means 6 are inputtedinto a coordinate converting means 100. The coordinate converting means100 obtains detection currents Iq and Id of two axes, a q axis and a daxis, respectively, from the motor position θ and the U-, V-, andW-phase detection currents by dq conversion. Also, a target currentcommand calculating means 1 of the motor calculates and outputs a targetq-axis current TIq and a target d-axis current TId that are specifiedcurrents for a motor drive.

A deviation between the target q-axis current TIq from the targetcurrent command calculating means 1 and an actually detected q-axiscurrent Iq is calculated by PI control by a proportional-integralcalculating means 2 that is a target voltage calculating means to obtaina command voltage Vq of the q axis. Likewise, a deviation between thetarget d-axis current TId and the detection d-axis current Id iscalculated by PI control to obtain a d-axis command voltage Vd.

The q-axis command voltage Vq and the d-axis command voltage Vdcalculated in the proportional-integral calculating means 2 are inputtedinto a three-phase conversion calculating means 3. The three-phaseconversion calculating means 3 performs three-phase conversion, that is,dq inverse conversion and thereby converts the d- and q-axis commandvoltages to voltage command values Vu, Vv, and Vw to be applied,respectively, to the armature windings of three phases U, V, and W ofthe motor.

The voltage command values Vu, Vv, and Vw are inputted into the PWMdriving means 4 that is a control means of the drive circuit. The PWMdriving means 4 substitutes duty ratios for the three-phase voltagecommand values and then applies pulse width modulation to provide driveinstructions to FET drive circuits. The FET drive circuits achievechopper control upon receipt of drive signals from the PWM driving means4. Consequently, currents are flown through the respective phases of thearmature windings 9 of the brushless motor and the brushless motorrotates by generating a torque.

A specific operation of current detection in the current detecting means6 u, 6 v, and 6 w will now be described with reference to FIG. 2. InFIG. 2, (1) denotes a timer indicating a carrier in pulse widthmodulation (PWM) and a duty ratio (2) is shown by setting a minimumstate to 0% and a maximum state to 100%.

When the carrier (1) exceeds the duty ratio (2), a signal (3) switchesON the FETs 5H disposed on the upper row whereas a signal (4) switchesOFF the FETs 5L disposed on the lower row. Conversely, when the carrier(1) drops below the duty ratio (2), the signal (3) switches OFF the FETs5H disposed on the upper row whereas the signal (4) switches ON the FETs5L disposed on the lower row.

By controlling an ON/OFF time ratio in this manner, voltages applied tothe armature windings 9 are controlled.

As is shown in FIG. 3, for the current detection, a state in which theupper FET is switched ON and the lower FET is switched OFF by thesignals (3) and (4) described above is defined as an OFF state, duringwhich current detection cannot be performed because currents flowingthrough the armature windings 9 do not pass through the currentdetecting means 6. Conversely, as is shown in FIG. 4, a state in whichthe upper FET is switched OFF and the lower FET is switched ON isdefined as an ON state, during which current detection can be performedbecause currents flowing through the armature windings 9 pass throughthe current detecting means 6. Hence, it is necessary to detect currentsin the current detection circuits during the ON state.

In the current detecting means 6 of FIG. 4, for a sample-and-holdinstructing signal to extract a current detection value alone, it isnecessary to hold and sample the current detection value during an ONstate, that is, within a time t1 of FIG. 2. Because the sample-and-holdtime t2 is determined by a sampling-and-holding means, t1≥t2 is requiredfor current detection to be performed.

More specifically, regarding evaluations as to whether or not a currentcan be detected, it is evaluated that a current can be detected when theON state period t1 is larger than t2 and it is evaluated that a currentcannot be detected when the ON state period t1 is shorter than t2.

The ON state period t1 indicates an ON time of the lower FET 5L, andgiven that a carrier cycle T of PWM is fixed, then t1 can be determinedfrom the duty ratio. Hence, a duty ratio with which a current can bedetected is t2/T or higher and it can be said that a state in which aduty ratio is t2/T or higher is a current detectable state. Conversely,a state in which the duty ratio is less than t2/T can be evaluated as acurrent undetectable state. Further, because duty ratios can be obtainedfrom command voltages (voltage command values) of the respective phases,it is possible to evaluate whether or not currents can be detected inthe respective phases from duty ratios that the respective phasesspecify or the command voltages of the respective phases.

Also, in the above description, whether or not currents can be detectedis evaluated from the specified duty ratios or the command voltages.However, voltages on the armature windings fluctuate to a power supplyvoltage and a ground voltage due to the upper and lower FETs 5H and 5L.Hence, whether or not currents can be detected may be evaluated bydetecting this voltage fluctuation to measure a time during which thevoltages on the armature windings are in a state on the ground side, andby evaluating whether the measured time is equal to or longer than t2.Further, whether or not currents can be detected may be evaluated bymeasuring a time during which the voltages on the armature windings arein a state on the ground side and a time during which the voltages arein a state on the power supply side and by calculating an actual dutyratio by obtaining a ratio of the measured times.

Furthermore, duty ratios can be calculated by detecting voltages on thearmature windings and a power supply voltage and by obtaining ratios ofthe former and the latter. Whether or not currents can be detected maybe evaluated from these duty ratios.

The coordinate converting means 100 that is a major portion of theinvention will now be described in detail on the basis of FIG. 5.Referring to FIG. 5, a current detection switching processing portion105 outputs instructing signals to switch the currents detected in therespective phases in addition to processing to evaluate whether or notcurrents can be detected in the respective phases as described above.When a current can be detected in the phase U, a U-phase current inputprocessing portion 101 outputs a detection current ADu inputted thereinintact as Iu. When it is evaluated that a current cannot be detected inthe phase U, Iu expressed by Equation (2) below is outputted:Iu=−(ADv+ADw)  (2).

The same applies to a V-phase current input processing portion 102, andwhen a current can be detected in the phase V, a detection current ADvinputted therein is outputted intact as Iv.

When it is evaluated that a current cannot be detected in the phase V,Iv expressed by Equation (3) below is outputted:Iv=−(ADu+ADw)  (3).

The same also applies to a W-phase current input processing portion 103,and when a current can be detected in the phase V, a detection currentADw inputted therein is outputted intact as Iw. When it is evaluatedthat a current cannot be detected in the phase W, Iw expressed byEquation (4) below is outputted:Iw=−(ADu+ADv)  (4).

In a case where there is only one phase in which a current can bedetected or currents cannot be detected in all the phases, the currentdetection switching processing portion 105 outputs phase currentscalculated before as currents in the respective phases.

Equation (5) below is calculated using the phase currents Iu, Iv, and Iwobtained from the respective phases as above and the d- and q-axisdetection currents denoted by Id and Iq, respectively, are outputted.

[Mathematical  Formula  1] $\begin{matrix}{\begin{bmatrix}{Id} \\{Iq}\end{bmatrix} = {{\sqrt{\frac{2}{3}}\begin{bmatrix}{\cos\;\theta} & {\cos\left( {\theta - {\frac{2}{3}\pi}} \right)} & {\cos\left( {\theta + {\frac{2}{3}\pi}} \right)} \\{{- \sin}\;\theta} & {{- \sin}\;\left( {\theta - {\frac{2}{3}\pi}} \right)} & {- {\sin\left( {\theta + {\frac{2}{3}\pi}} \right)}}\end{bmatrix}}\begin{bmatrix}{Iu} \\{Iv} \\{Iw}\end{bmatrix}}} & (5)\end{matrix}$

The processing above is depicted by the flowchart of FIG. 6.

In Step S1, a duty ratio of the phase U, Du, is compared with t2/T toevaluate whether or not a current can be detected. When a current can bedetected, the flow proceeds to Step S2. When it is evaluated that acurrent cannot be detected, the flow proceeds to Step S3.

In Step S2, a duty ratio of the phase V, Dv, is compared with t2/T toevaluate whether or not a current can be detected. When a current can bedetected, the flow proceeds to Step S4. When it is evaluated that acurrent cannot be detected, the flow proceeds to Step S5.

In Step S3, the duty ratio of the phase V, Dv, is compared with t2/T toevaluate whether or not a current can be detected. When a current can bedetected, the flow proceeds to Step S6. When it is evaluated that acurrent cannot be detected, the flow proceeds to Step S13.

In Step S4, a duty ratio of the phase W, Dw, is compared with t2/T toevaluate whether or not a current can be detected. When a current can bedetected, the flow proceeds to Step S7. When it is evaluated that acurrent cannot be detected, the flow proceeds to Step S8.

In Step S5, the duty ratio of the phase W, Dw, is compared with t2/T toevaluate whether or not a current can be detected. When a current can bedetected, the flow proceeds to Step S9. When it is evaluated that acurrent cannot be detected, the flow proceeds to Step S10.

In Step S6, the duty ratio of the phase W, Dw, is compared with t2/T toevaluate whether or not a current can be detected. When a current can bedetected, the flow proceeds to Step S11. When it is evaluated that acurrent cannot be detected, the flow proceeds to Step S12.

In Step S7, because currents in all the three phases can be detected,signals ADu, Adv, and ADw detected by the current detecting means areused intact.

In Step S8, because a current in the phase W cannot be detected, acurrent in the phase W is calculated in accordance with Equation (4)above. For currents in the other phases U and V, signals ADu and ADvdetected by the current detecting means are used intact.

In Step S9, because a current in the phase V cannot be detected, acurrent in the phase V is calculated in accordance with Equation (3)above. For currents in the other phases U and W, signals ADu and ADwdetected by the current detecting means are used intact.

In Step S10, it is found that currents in two phases cannot be detected,that is, a current only in one phase can be detected. Hence, last valuesof the detection currents in the respective phases are used.

In Step S11, because a current in the phase U cannot be detected, acurrent in the phase U is calculated in accordance with Equation (2)above. For currents in the other phases V and W, signals ADv and ADwdetected by the current detecting means are used intact.

In Step S12, it is found that current in two phases cannot be detected,that is, a current only in one phase can be detected. Hence, last valuesof the detection currents in the respective phases are used.

In Step S13, it is found that currents in two or more phases cannot bedetected, that is, currents in two phases or in all the phases cannot bedetected. Hence, last values of the detection currents in the respectivephases are used.

In Step S15, Flg=0 is set to indicate a state in which currents can bedetected.

In Step S16, Flg=1 is set to indicate the presence of a state in which acurrent cannot be detected.

In Step S17, Equation (5) above is calculated from detection currents inthe three phases.

As has been described, a brushless-motor drive apparatus of the firstembodiment is furnished with: a drive circuit of a brushless motorhaving a plurality of parallel-connected arms including switchingelement pairs formed of first (upper) switching elements disposed on apower supply side and second (lower) switching elements disposed on aground side connected in series in each pair; a current detecting meansfor detecting currents flowing through armature windings of thebrushless motor during periods when the switching elements of the drivecircuit are ON; a calculation processing means for comparing a targetcurrent value to drive the brushless motor and detection current valuesdetected by the current detecting means, and calculating voltage commandvalues to be applied to the armature windings of the brushless motor onthe basis of deviations; and a PWM driving means for controlling ON/OFFof the switching elements of the drive circuit by generating drivesignals on the basis of the voltage command values from the calculationprocessing means. The calculation processing means is further providedwith a current-detection possibility evaluating means for evaluatingwhether or not the currents flowing through the armature windings ofrespective phases of the brushless motor can be detected according tooperating states of the switching elements of the respective phases ofthe drive circuit. When the current-detection possibility evaluatingmeans evaluates that the currents cannot be detected, the calculationprocessing means obtains the voltage command values by making acomparison with the target current value using detection current valuesdetected before and continues a motor drive. Hence, there is no need tolimit duty ratios of the respective phases to a range within which thecurrents can be detected. Consequently, duty ratios can be used to thegreatest extent possible.

Also, the brushless motor has armature windings formed of n phases, andwhen the current-detection possibility evaluating means evaluates thatphases in which currents can be detected are (n−2) phases or less, thecalculation processing means obtains the voltage command values bycomparing phase current detection values and the target current valueusing phase current detection values obtained when all phase currentsare detected and continues the motor drive. Hence, for example, even ina case where a current in only one phase can be detected and currents inthe respective phases cannot be estimated, it becomes possible to drivethe motor independently of a state in which currents cannot be detected.Accordingly, a need to limit duty ratios can be eliminated.

Incidentally, in a circumstance where a current cannot be detected,command voltages of the phases U, V, and W are required to increase withan increase of target voltages of the d- and q-axes. In other words, ina circumstance where a voltage rise occurs in the armature windings ofthe motor, the motor rotates and a back electromotive force is generatedaccording to a rotation speed, so that inductive voltages in therespective phases increase. Hence, a circumstance where currents cannotbe detected due to a failure of the motor drive apparatus can be avoidedby detecting a rotation state of the motor.

In other words, in a circumstance where currents cannot be detected,when the motor rotation speed is equal to or below a predeterminedvalue, the motor drive is stopped. Owing to this configuration, itbecomes possible to stop the motor drive by detecting an abnormality inthe motor drive apparatus.

The content described above is processed in the PWM driving means 4 onthe basis of the processing in FIG. 6, and the processing of the contentis depicted by the flowchart of FIG. 7.

Referring to FIG. 7, the current undetectable state flag Flg obtained bythe processing in FIG. 6 is checked. When the flag exhibits a currentdetectable state, the flow proceeds to Step S34. When the flag exhibitsa current undetectable state, the flow proceeds to Step S32.

In Step S32, whether or not the motor rotation speed is equal to orhigher than a predetermined value a is evaluated. When the motorrotation speed is equal to or higher than the predetermined value, theflow proceeds to Step S34. When the motor rotation speed is below thepredetermined value α, the flow proceeds to Step S33.

In Step S33, an abnormality is evaluated and it is instructed to stopPWM control.

In Step S34, the specified respective phase voltages are converted toduties to enable PWM control.

In Step S35, in order to switch ON or OFF the FETs of the motor drivecircuit, control signals of an ON/OFF instruction according to PWMcontrol or control signals to switch OFF all the FETs to stop PWMcontrol are outputted.

It is known that the motor normally has a characteristic M shown in FIG.8. Hence, in order to improve detection accuracy of an abnormality, athreshold TH determined by the motor rotation speed and the q-axiscurrent is obtained in advance, and it is evaluated whether the motor isin a region A greater than the threshold TH or a region B smaller thanthe threshold TH on the basis of the motor rotation speed obtained froma detected motor angle and the detected q-axis current, and the motordrive is stopped when it is found that the motor is in the region B andin a circumstance where currents cannot be detected.

Referring to FIG. 7, it is evaluated in Step S32 whether or not themotor rotation speed is below the predetermined value. However, as isshown in FIG. 8, this evaluation can be made by evaluating whether theregion specified by the q-axis current and the motor rotation speed isthe region A or B and the flow is allowed to proceed to Step S34 whenthe region is the region A and to Step S33 when the region is the regionB.

As has been described above, the brushless-motor drive apparatusaccording to the first embodiment of the invention is configured in sucha manner that when a motor angle is a predetermined motor angle, whenthe motor rotation speed is equal to or below a predetermined motorrotation speed and the current-detection possibility evaluating meansevaluates that phase currents of the armature windings cannot bedetected, or when the motor is within a range of a predetermined motorcharacteristic and the current-detection possibility evaluating meansevaluates that phase currents cannot be detected, the PWM driving meansstops the motor drive by switching OFF all the switching elements.Hence, even in a state in which currents cannot be detected, anabnormality in the motor drive apparatus can be detected and the motordrive can be stopped.

Second Embodiment

FIG. 9 shows another method of the coordinate converting means 100according to a second embodiment of the invention. Referring to FIG. 9,on the assumption that currents can be detected in all the phases, a U-,V-, and W-phase coordinate conversion processing portion 111 calculatesEquation (6) below using phase currents ADu, ADv, and ADw obtained fromthe respective phases and outputs d- and q-axis detection currentsdenoted by Id and Iq, respectively.

[Mathematical  Formula  2] $\begin{matrix}{\begin{bmatrix}{Id} \\{Iq}\end{bmatrix} = {{\sqrt{\frac{2}{3}}\begin{bmatrix}{\cos\;\theta} & {\cos\left( {\theta - {\frac{2}{3}\pi}} \right)} & {\cos\left( {\theta + {\frac{2}{3}\pi}} \right)} \\{{- \sin}\;\theta} & {{- \sin}\;\left( {\theta - {\frac{2}{3}\pi}} \right)} & {- {\sin\left( {\theta + {\frac{2}{3}\pi}} \right)}}\end{bmatrix}}\begin{bmatrix}{ADu} \\{ADv} \\{Adw}\end{bmatrix}}} & (6)\end{matrix}$

On the assumption that a current in the phase U cannot be detected, a V-and W-phase coordinate conversion processing portion 114 calculatesEquation (7) below using phase currents ADv and ADw obtained from therest of two phases, and outputs d- and q-axis detection currents denotedby Id and Iq, respectively.

[Mathematical  Formula  3] $\begin{matrix}{\begin{bmatrix}{Id} \\{Iq}\end{bmatrix} = {{\sqrt{2}\begin{bmatrix}{\sin\left( {\theta - \frac{\pi}{3}} \right)} & {- {\sin\left( {\theta + \frac{\pi}{3}} \right)}} \\{\cos\;\left( {\theta - \frac{\pi}{3}} \right)} & {- {\cos\left( {\theta + \frac{\pi}{3}} \right)}}\end{bmatrix}}\begin{bmatrix}{ADv} \\{ADw}\end{bmatrix}}} & (7)\end{matrix}$

On the assumption that a current in the phase V cannot be detected, a U-and W-phase coordinate conversion processing portion 113 calculatesEquation (8) below using phase currents ADu and ADw obtained from therest of two phases, and outputs d- and q-axis detection currents denotedby Id and Iq, respectively.

[Mathematical  Formula  4] $\begin{matrix}{\begin{bmatrix}{Id} \\{Iq}\end{bmatrix} = {{\sqrt{2}\begin{bmatrix}{- {\sin\left( {\theta - \frac{\pi}{3}} \right)}} & {{- \sin}\;\theta} \\{{- \cos}\;\left( {\theta - \frac{\pi}{3}} \right)} & {{- \cos}\;\theta}\end{bmatrix}}\begin{bmatrix}{ADu} \\{ADw}\end{bmatrix}}} & (8)\end{matrix}$

On the assumption that a current in the phase W cannot be detected, a U-and V-phase coordinate conversion processing portion 112 calculatesEquation (9) below using phase currents ADu and ADv obtained from therest of two phases, and outputs d- and q-axis detection currents denotedby Id and Iq, respectively.

[Mathematical  Formula  5] $\begin{matrix}{\begin{bmatrix}{Id} \\{Iq}\end{bmatrix} = {{\sqrt{2}\begin{bmatrix}{\sin\left( {\theta + \frac{\pi}{3}} \right)} & {\sin\;\theta} \\{\cos\;\left( {\theta + \frac{\pi}{3}} \right)} & {\cos\;\theta}\end{bmatrix}}\begin{bmatrix}{ADu} \\{ADv}\end{bmatrix}}} & (9)\end{matrix}$

In a case where currents in the three phases can be detected, a d- andq-axis current switching processing portion 110 selects the d- andq-axis currents Id and Iq obtained in the U-, V-, and W-phase coordinateconversion processing portion 111.

In a case where a current in the phase U alone cannot be detected, thed- and q-axis currents Id and Iq obtained in the V- and W-phasecoordinate conversion processing portion 114 are selected.

In a case where a current in the phase V alone cannot be detected, thed- and q-axis currents Id and Iq obtained in the U- and W-phasecoordinate conversion processing portion 113 are selected.

In a case where a current in the phase W alone cannot be detected, thed- and q-axis currents Id and Iq obtained in the V- and W-phasecoordinate conversion processing portion 112 are selected. In a casewhere currents in two phases or all the phases cannot be detected,coordinate conversion to Id and Iq is not performed and last values areused.

The processing above is depicted by the flowchart of FIG. 10.

In Step S121, a duty ratio of the phase U, Du, is compared with t2/T toevaluate whether or not a current can be detected. When a current can bedetected, the flow proceeds to Step S122. When it is evaluated that acurrent cannot be detected, the flow proceeds to Step S123.

In Step S122, a duty ratio of the phase V, Dv, is compared with t2/T toevaluate whether or not a current can be detected. When a current can bedetected, the flow proceeds to Step S124. When it is evaluated that acurrent cannot be detected, the flow proceeds to Step S125.

In Step S123, the duty ratio of the phase V, Dv, is compared with t2/Tto evaluate whether or not a current can be detected. When a current canbe detected, the flow proceeds to Step S126. When it is evaluated that acurrent cannot be detected, the flow proceeds to Step S137.

In Step S124, a duty ratio of the phase W, Dw, is compared with t2/T toevaluate whether or not a current can be detected. When a current can bedetected, the flow proceeds to Step S127. When it is evaluated that acurrent cannot be detected, the flow proceeds to Step S129.

In Step S125, the duty ratio of the phase W, Dw, is compared with t2/Tto evaluate whether or not a current can be detected. When a current canbe detected, the flow proceeds to Step S131. When it is evaluated that acurrent cannot be detected, the flow proceeds to Step S133.

In Step S126, the duty ratio of the phase W, Dw, is compared with t2/Tto evaluate whether or not a current can be detected. When a currentcannot be detected, the flow proceeds to Step S134. When it is evaluatedthat a current cannot be detected, the flow proceeds to Step S136.

In Step S127, because currents in all the three phases can be detected,signals ADu, ADv, and ADw detected by the current detecting means areused intact.

In Step S128, Equation (6) above is calculated.

In Step S129, because a current in the phase W cannot be detected,signals ADu and ADv detected by the current detecting means are usedintact for the phases U and V.

In Step S130, Equation (9) above is calculated.

In Step S131, because a current in the phase V cannot be detected,signals ADu and ADw detected by the current detecting means are usedintact for the other phases U and W.

In Step S132, Equation (8) above is calculated.

In Step S133, it is found that currents in two phases cannot bedetected, that is, a current only in one phase can be detected. Hence,last values of the d- and q-axis detection currents are used.

In Step S134, because a current in the phase U cannot be detected,signals ADv and ADw detected by the current detecting means are usedintact for the other phases V and W.

In Step S135, Equation (7) above is calculated.

In Step S136, it is found that currents in two phases cannot bedetected, that is, a current only in one phase can be detected. Hence,last values of the d- and q-axis detection currents are used.

In Step S137, it is found that currents in two or more phases cannot bedetected, that is, currents in two phases or all phases cannot bedetected. Hence, last values of the d- and q-axis detection currents areused.

In Step S141, because it is a state in which currents in all phases canbe detected, Flg=0 is set.

In Step S142, because it is a state in which a current in the phase Wcannot be detected, Flg=3 is set.

In Step S143, because it is a state in which a current in the phase Vcannot be detected, Flg=2 is set.

In Step S144, because it is a state in which currents in two or morephases cannot be detected, Flg=4 is set.

In Step S145, because it is a state in which a current in the phase Ucannot be detected, Flg=1 is set.

In Step S146, because it is a state in which currents in two or morephases cannot be detected, Flg=4 is set.

In Step S147, because it is a state in which currents in two or morephases cannot be detected, Flg=4 is set.

As has been described, according to the second embodiment of theinvention, the brushless-motor has armature windings formed of threephases U, V, and W. The the calculation processing means includes: afirst coordinate converting means for converting the detection currentsdetected by the current detecting means to a biaxial current made up ofa d-axis current indicating a current in a magnetic flux direction and aq-axis current indicating a current in a torque direction; a targetvoltage calculating means for obtaining a biaxial target voltage from abiaxial target current value and a biaxial detection current valueobtained by the first coordinate converting means; and a three-phaseconverting means (second coordinate converting means) for converting thebiaxial target voltage obtained by the target voltage calculating meansto voltage command values of three phases. When the current-detectionpossibility evaluating means evaluates that currents in all three phasesflowing through the armature windings of the respective phases can bedetected, detection currents in the phases U, V, and W are converted tothe biaxial current by the first coordinate converting means. When thecurrent-detection possibility evaluating means evaluates that a currentin one phase cannot be detected, the current in the phase in which thecurrent cannot be detected is obtained from currents in the other twophases in which the currents can be detected, and the detection currentsin the phases U, V, and Ware converted to the biaxial current by thefirst coordinate converting means. When the current-detectionpossibility evaluating means evaluates that currents in two or morephases cannot be detected, the d-axis and q-axis currents obtained bythe first coordinate converting means last time are used for acalculation of the voltage command values. Hence, even in a case where acurrent only in one phase can be detected and respective phase currentscannot be estimated, it becomes possible to calculate detection valuesof the respective phase currents and convert the detection values to abiaxial current. Accordingly, because there is no need to limit dutyratios of the respective phases to a range within which currents can bedetected, duty ratios can be used to the greatest extent possible.

Also, because a calculation can be carried out by also performing acoordinate conversion, it becomes possible to reduce a processing loadon the calculation processing means.

Incidentally, when voltages applied to the armature windings, that is tosay, amplitude of the duty ratios increase, currents can no longer bedetected but not uniformly. Because voltages act on the armaturewindings depending on the phases, a region in which a current can nolonger be detected varies with the motor angle. As is shown in FIG. 11,when amplitude of the duty ratios is small as in a region A, currentscan be detected in all the phases.

When amplitude of the duty ratios increases, the circumstance changes toa circumstance where a currents in one phase cannot be detected inregions B through D. In this instance, a phase in which a current cannotbe detected is different in each of the regions B, C, and D.

When amplitude of the duty ratios increases further, currents in twophases can no longer be detected in a region E.

Hence, by confirming a relation between a state in which a currentcannot be detected as shown in FIG. 11 and a motor angle in such astate, it becomes possible to detect an abnormality in the motor driveapparatus.

The content above is processed in the PWM driving means 4 and thisprocessing is depicted by the flowchart of FIG. 12.

Referring to FIG. 12, in Step S150, specified voltages of the respectivephases are converted to duties.

In Step S151, the current undetectable state present flag Flg obtainedin the processing of FIG. 10 is checked. When Flg exhibits 0, the flowproceeds to Step S152, and when Flg exhibits 1, 2, 3, and 4, the flowproceeds to Step S153, Step S154, Step S155, and Step S156,respectively.

In Step S152, whether the duty ratios are in the region A is evaluatedfrom the motor angle. When the duty ratios are in the region A, the flowproceeds to Step S158; otherwise, the flow proceeds to Step S157.

In Step S153, whether the duty ratios are in the region B is evaluatedfrom the motor angle. When the duty ratios are in the region B, the flowproceeds to Step S158; otherwise, the flow proceeds to Step S157.

In Step S154, whether the duty ratios are in the region C is evaluatedfrom the motor angle. When the duty ratios are in the range C, the flowproceeds to Step S158; otherwise, the flow proceeds to Step S157.

In Step S155, whether the duty ratios are in the region D is evaluatedfrom the motor angle. When the duty ratios are in the range D, the flowproceeds to Step S158; otherwise, the flow proceeds to Step S157.

In Step S156, whether the duty ratios are in the region E is evaluatedfrom the motor angle. When the duty ratios are in the region E, the flowproceeds to Step S158; otherwise, the flow proceeds to Step S157.

In Step S157, the duty ratios obtained in Step S150 are limited and setto be t2/T or higher. With these duties, the duty ratios can be forcedlyswitched to a region in which currents can be detected. In other words,it becomes possible to detect currents by changing PWM drive signals soas to extend an ON time of the lower FETs shown in the first embodimentabove.

Accordingly, it becomes possible to detect currents, and an abnormalitycan be evaluated by detecting the currents. Also, as with the methoddescribed in the first embodiment above, there is no need to stop themotor drive, and although voltages to be applied to the armaturewindings are reduced, the motor drive can be continued. Also, in a casewhere the duty ratios are limited, duty ratios may be limited phase byphase or it becomes possible to detect currents in all the phases bylimiting the duty ratios of all the phases at the same time.

In Step S158, the duty ratios are outputted to the FET drive circuits asPWM control signals to drive the FET drive circuits to switch ON/OFF.

As has been described in the first embodiment above, the motor isrotating when currents cannot be detected.

Hence, the motor angle keeps varying and the regions shown in FIG. 11change constantly. In other words, a state in which currents cannot bedetected does not last long, and a current detectable state and acurrent undetectable state change from one to the other with a motorrotation speed.

Hence, instead of evaluating an abnormality from the motor angle, it maybe configured in such a manner that a time over which a currentundetectable state continues is measured and a predetermined value, forexample, a time corresponding to a motor rotation speed is set, so thatan abnormality is evaluated when the current undetectable state hascontinued for the set time or more.

Steps S152 through S156 of the flowchart shown in FIG. 12 to evaluate anabnormality may be replaced by processing to check whether respectivestates continue for a predetermined time, and this replacement isdepicted by the flowchart of FIG. 13. As is shown in FIG. 13, even whenthe abnormality evaluating steps are replaced by Steps S161 throughS164, the same function and advantage can be achieved.

As has been described, according to the second embodiment of theinvention, when a state in which currents cannot be detected continuesfor a predetermined time in the current-detection possibility evaluatingmeans, the PWM driving means outputs a drive signal that extends an ONperiod of the second switching element of a phase in which a currentcannot be detected so that a phase current can be detected. Hence, in astate in which the motor drive is continued using last detectioncurrents, it becomes possible to prevent a state in which an abnormalityof the motor drive apparatus cannot be detected.

Also, when a state in which currents cannot be detected continues for asecond predetermined time in the current-detection possibilityevaluation means, the PWM driving means outputs drive signals thatextend an ON period of the second switching elements of all the phases.Hence, even when the motor drive apparatus fails, not only does itbecome possible to prevent a state in which a failure cannot bedetected, but it also become possible to perform the motor drivesmoothly by lowering all outputs.

Also, it may be configured in such a manner that a means for detectingan angle of the magnetic pole position of the brushless motor isprovided, and in a case where it is evaluated by the current-detectionpossibility evaluating means that phase currents cannot be detected whenthe motor angle is a predetermined angle, duty ratios are limited as inStep S157 on the same ground for the evaluation described in Steps S161through S164 of FIG. 13, so that drive signals that extend an ON periodof the second switching elements are outputted. Even when configured inthis manner, the same advantage can be achieved.

Further, it may be configured in such a manner that a means forcalculating a motor rotation speed by angle detection is provided, andin a case where it is evaluated by the current-detection possibilityevaluation means that phase currents cannot be detected when the motorrotation speed is equal to or below a predetermined motor rotationspeed, the PWM driving means outputs drive signals that extend an ONperiod of all the second switching elements. Even when configured inthis manner, too, not only does it become possible to prevent a state inwhich an abnormality of the motor drive apparatus cannot be detected,and it also becomes possible to perform the motor drive smoothly bylowering all outputs.

Furthermore, it may be configured in such a manner that in a case whereit is evaluated by the current-detection possibility evaluating meansthat phase currents cannot be detected when the motor is within a rangeof a predetermined motor characteristic, the PWM driving means outputsdrive signals that extend an ON period of all the second switchingelements. When configured in this manner, the same advantages can beachieved.

INDUSTRIAL APPLICABILITY

The invention provides a brushless-motor drive apparatus suitably usedfor example, for an electrical power steering apparatus installed to avehicle.

DESCRIPTION OF NUMERAL REFERENCES AND SIGNS

1: target current command calculating means

2: target voltage calculating means (PI control calculating means)

3: three-phase conversion calculating means

4: PWM driving means

5H (5 uH, 5 vH, and 5 wH): first switching elements (FETs)

5L (5 uL, 5 vL, and 5 wL): second switching elements (FETs)

6 (6 u, 6 v, and 6 w): current detecting means

9: armature windings of brushless motor

10: position sensor

100: coordinate converting means

The invention claimed is:
 1. A brushless-motor drive apparatuscomprising: a drive circuit of a brushless motor, the drive circuitincluding switching element pairs formed of first switching elementsdisposed on a power supply side and second switching elements disposedon a ground side connected in series in each pair; a current detectorconfigured to detect currents flowing through armature windings of thebrushless motor during periods when the second switching elements areON; a calculation processor configured to compare a target current valueto drive the brushless motor and current values of the currents detectedby the current detector, and calculate voltage command values to beapplied to the armature windings of the brushless motor based ondeviations between the target current value and the current values ofthe detected currents; and a PWM driver configured to control ON and OFFswitching of the first and second switching elements by generating drivesignals based on the voltage command values, wherein the calculationprocessor includes a current-detection possibility evaluator configuredto evaluate whether the currents flowing through the armature windingsof respective phases of the brushless motor can be detected according tooperating states of the first and second switching elements for therespective phases, when the current-detection possibility evaluatorevaluates that the currents cannot be detected, the calculationprocessor is further configured to obtain the voltage command values bycomparing the target current value with the current values which havebeen detected before and continue a motor drive, and the currentdetector comprises resistors, each of which is connected between arespective phase of three phases of the brushless motor and a ground,wherein, in response to the evaluating that the currents in all of thethree phases can be detected, the calculation processor calculates thevoltage command values, to be applied to the armature windings, from thedetected currents in all of the three phases, in response to theevaluating that the current in one of the three phases cannot bedetected, the calculation processor obtains the current in the one ofthe three phases in which the current cannot be detected from thedetected currents in other two phases, of the three phases, in which thecurrents can be detected, and calculates the voltage command values, tobe applied to the armature windings, from the obtained current of theone of the three phases and the detected currents in the other twophases, and in response to the evaluating that the currents in at leasttwo phases of the three phases cannot be detected, the calculationprocessor calculates the voltage command values, to be applied to thearmature windings, based on the deviations between the target currentvalue and the current values having been detected in the at least twophases in a last current detection cycle and stored.
 2. Thebrushless-motor drive apparatus according to claim 1, wherein thebrushless motor has the armature windings formed of phases U, V, and W,and the calculation processor further includes: a first coordinateconverter configured to convert the detected currents to a biaxialcurrent made up of a d-axis current indicating a current in a magneticflux direction and a q-axis current indicating a current in a torquedirection; a target voltage calculator configured to obtain a biaxialtarget voltage from a biaxial target current value and a biaxialdetection current value obtained by the first coordinate converter; anda three-phase converter configured to convert the biaxial target voltageto the voltage command values of the three phases, wherein, when thecurrent-detection possibility evaluator evaluates that the currents inall three phases can be detected, the detected currents in the phases U,V, and W are converted to the biaxial current by the first coordinateconverter, when the current-detection possibility evaluator evaluatesthat the current in the one of the three phases cannot be detected, thecurrent in the one of the three phases in which the current cannot bedetected is obtained from the currents in the other two phases in whichthe currents can be detected, and the obtained current of the one of thethree phases and the detected currents in the other two phases areconverted to the biaxial current by the first coordinate converter, andwhen the current-detection possibility evaluator evaluates that thecurrents in the at least two phases cannot be detected, the d-axiscurrent and the q-axis current obtained by the first coordinateconverter in the last current detection cycle are used for a calculationof the voltage command values.
 3. The brushless-motor drive apparatusaccording to claim 2, wherein: when the current-detection possibilityevaluator evaluates that the currents in all three phases can bedetected, the calculation processor converts the detected currents inthe phases U, V, and W to the biaxial current using the first coordinateconverter; when the current-detection possibility evaluator evaluatesthat a current in the phase U cannot be detected, the calculationprocessor converts the detected currents in the phases V and W to thebiaxial current; when the current-detection possibility evaluatorevaluates that a current in the phase V cannot be detected, thecalculation processor converts the detected currents in the phases U andW to the biaxial current; when the current-detection possibilityevaluator evaluates that a current in the phase W cannot be detected,the calculation processor converts the detected currents in the phases Uand V to the biaxial current; and when the current-detection possibilityevaluator evaluates that the currents in the at least two phases cannotbe detected, the d-axis current and the q-axis current obtained by lastcoordinate conversion are used.
 4. The brushless-motor drive apparatusaccording to claim 1, wherein: when a state in which the currents cannotbe detected continues for a predetermined time, the PWM driver outputs adrive signal that extends an ON period of the second switching elementof a phase in which a current cannot be detected, so that a phasecurrent can be detected.
 5. The brushless-motor drive apparatusaccording to claim 1, wherein: when a state in which the currents cannotbe detected continues for a second predetermined time, the PWM driver isconfigured to output drive signals that extend an ON period of thesecond switching elements of all of the phases.
 6. The brushless-motordrive apparatus according to claim 1, further comprising: an angledetector configured to detect an angle of a magnetic pole position ofthe brushless motor, wherein, in a case where the current-detectionpossibility evaluator evaluates that phase currents cannot be detectedwhen the angle is a predetermined motor angle, the PWM driver isconfigured to output a drive signal that extends an ON period of thesecond switching element of a phase in which a current cannot bedetected, so that a phase current can be detected.
 7. Thebrushless-motor drive apparatus according to claim 1, furthercomprising: an angle detector configured to detect an angle of amagnetic pole position of the brushless motor, wherein, in a case wherethe current-detection possibility evaluator evaluates that phasecurrents cannot be detected when the angle is a predetermined motorangle, the PWM driver is configured to output drive signals that extendan ON period of all of the second switching elements.
 8. Thebrushless-motor drive apparatus according to claim 1, furthercomprising: a motor rotation speed calculator configured to calculate amotor rotation speed based on an angle detection, wherein, in a casewhere the current-detection possibility evaluator evaluates that phasecurrents cannot be detected when the motor rotation speed is equal to orbelow a predetermined rotation speed, the PWM driver is configured tooutput drive signals that extend an ON period of all of the secondswitching elements.
 9. The brushless-motor drive apparatus according toclaim 1, wherein: in a case where the current-detection possibilityevaluator evaluates that phase currents cannot be detected when thebrushless motor is within a range of a predetermined motorcharacteristic, the PWM driver is configured to output drive signalsthat extend an ON period of all of the second switching elements. 10.The brushless-motor drive apparatus according to claim 1, wherein: whena state in which the currents cannot be detected continues for a secondpredetermined time, the PWM driver is configured to stop the motor driveby switching OFF the first and second switching elements.
 11. Thebrushless-motor drive apparatus according to claim 1, furthercomprising: an angle detector configured to detect an angle of magneticpole position of the brushless motor, wherein, in a case where thecurrent-detection possibility evaluator evaluates that phase currentscannot be detected when the angle is a predetermined motor angle, thePWM driver is configured to stop the motor drive by switching OFF thefirst and second switching elements.
 12. The brushless-motor driveapparatus according to claim 1, further comprising: a motor rotationspeed calculator configured to calculate a motor rotation speed based onan angle detection, wherein, in a case where the current-detectionpossibility evaluator evaluates that phase currents cannot be detectedwhen the motor rotation speed is equal to or below a predeterminedrotation speed, the PWM driver is configured to stop the motor drive byswitching OFF the first and second switching elements.
 13. Thebrushless-motor drive apparatus according to claim 1, wherein: in a casewhere the current-detection possibility evaluator evaluates that phasecurrents cannot be detected when the brushless motor is within a rangeof a predetermined motor characteristic, the PWM driver is configured tostop the motor drive by switching OFF the first and second switchingelements.
 14. The brushless-motor drive apparatus according to claim 1,wherein the calculation processor is configured to perform a currentfeedback control by calculating the voltage command values to be appliedto the armature windings based on the deviations between the targetcurrent value and the current values for the respective phases detectedby individual resistors of the current detector.
 15. A brushless-motordrive apparatus comprising: a drive circuit of a brushless motor, thedrive circuit including switching element pairs formed of firstswitching elements disposed on a power supply side and second switchingelements disposed on a ground side connected in series in each pair; acurrent detector configured to detect currents flowing through armaturewindings of the brushless motor during periods when the second switchingelements are ON; a calculation processor configured to compare a targetcurrent value to drive the brushless motor and current values of thecurrents detected by the current detector, and calculate voltage commandvalues to be applied to the armature windings of the brushless motorbased on deviations between the target current value and the currentvalues of the detected currents; and a PWM driver configured to controlON and OFF switching of the first and second switching elements bygenerating drive signals based on the voltage command values, whereinthe calculation processor includes a current-detection possibilityevaluator configured to evaluate whether the currents flowing throughthe armature windings of respective phases of the brushless motor can bedetected according to operating states of the first and second switchingelements for the respective phases, when the current-detectionpossibility evaluator evaluates that the currents cannot be detected,the calculation processor is further configured to obtain the voltagecommand values by comparing the target current value with the currentvalues which have been detected before and continue a motor drive, andthe current detector comprises resistors, each of which is connectedbetween a respective phase of three phases of the brushless motor and aground, wherein, in response to the evaluating that the currents in allof the three phases can be detected, the calculation processorcalculates the voltage command values, to be applied to the armaturewindings, from the detected currents in all of the three phases, inresponse to the evaluating that the current in one of the three phasescannot be detected, the calculation processor obtains the current in theone of the three phases in which the current cannot be detected from thedetected currents in other two phases, of the three phases, in which thecurrents can be detected, and calculates the voltage command values, tobe applied to the armature windings, from the obtained current of theone of the three phases and the detected currents in the other twophases, and in response to the evaluating that the currents in at leasttwo phases of the three phases cannot be detected, the calculationprocessor calculates the voltage command values, to be applied to thearmature windings, based on the current values which have been detectedin the at least two phases in a last detection cycle and held, andwherein the brushless-motor drive apparatus further comprises: an angledetector configured to detect an angle of a magnetic pole position ofthe brushless motor; and a motor rotation speed calculator configured tocalculate a motor rotation speed based on the detected angle, wherein,when the current-detection possibility evaluator evaluates that thecurrent in at least one of the three phases cannot be detected while thedetected angle is a predetermined motor angle or the motor rotationspeed is equal to or below a predetermined rotation speed, the PWMdriver is configured to output drive signals that extend an ON period inthe second switching element of the at least one of the three phases.